Stressing that the “world is watching Syria,” Mr Ban said the UN experts must be allowed to conduct a “full, thorough and unimpeded” investigation.

“We cannot allow impunity in what appears to be a grave crime against humanity,” he said.

Every hour counts. We cannot afford any more delays. The world is watching Syria.

Ban Ki Moon

“If proven, any use of chemical weapons by anyone under any circumstances is a serious violation of international law and an outrageous crime,” he said.

Prime Minister Kevin Rudd says the UN inspectors will be critical in determining what weapons have been used and ultimately who is responsible.

However, he has raised concerns that by the time inspectors reach the site, they may have trouble finding evidence of a chemical attack.

“The more time elapses with the use of a nerve gas like sarin, its presence in the soil or ground degrades over time and makes it harder to detect,” he said.

“However when it’s been the cause of people’s deaths, there are still opportunities to establish through toxicology whether sarin’s been used.”

UN investigation will not apportion blame

Former Army officer and UN military advisor Dr Rodger Shanahan has served in Syria.

He says if the UN inspectors do find evidence that an attack has occurred, the next question is who ordered it.

“You would think that president Assad would know best of all the consequences of ordering such a strike,” he said.

“It could be somebody in a chain of command who did it without reference to Assad or Assad’s inner circle.

“These are the kinds of issues that we really need to get to the bottom of.”

The UN team is expected to collect forensic evidence including tissue samples as well as fragments from rockets that may have transported the gas.

But Dr Shanahan warns that the UN mandate is limited.

“It allows them to determine if an attack occurred, but doesn’t allow them to apportion blame,” he said.

He says knowledge of the weapon type along with analysis of the craters caused by the rockets, will allow inspectors to draw conclusions about who actually fired the weapons.

Probe is too late to be credible, says US

Earlier remarks from the US that the investigation is too late appear to signal that a military response is more likely.

Senator Bob Corker, the ranking Republican on the Foreign Affairs Committee, said he had spoken to the Obama administration about its plans and believed the president would seek authorisation for intervention after Congress convenes on September 9.

“I think we will respond in a surgical way,” he told Fox News on Sunday.

Mr Obama called British prime minister David Cameron on Saturday to discuss options, after a meeting of all his top diplomatic, military and intelligence chiefs at the White House.

US defence secretary Chuck Hagel said the military would be ready to act if called on by Mr Obama.

“President Obama has asked the Defence Department to prepare options for all contingencies. We have done that,” Mr Hagel said.

The comments follow forceful remarks from other Western powers, including Britain and France, which also believe Assad’s government is behind last week’s attack.

Russia, however, has warned the West against launching military action against the Syrian regime.

The UN says more than 100,000 people have been killed in Syria since an uprising against Assad’s rule flared in March 2011.